Integral vessel isolation valve

ABSTRACT

A nuclear reactor comprises a nuclear reactor core disposed in a pressure vessel. An isolation valve protects a penetration through the pressure vessel. The isolation valve comprises: a mounting flange connecting with a mating flange of the pressure vessel; a valve seat formed into the mounting flange; and a valve member movable between an open position and a closed position sealing against the valve seat. The valve member is disposed inside the mounting flange or inside the mating flange of the pressure vessel. A biasing member operatively connects to the valve member to bias the valve member towards the open position. The bias keeps the valve member in the open position except when a differential fluid pressure across the isolation valve and directed outward from the pressure vessel exceeds a threshold pressure.

This application is a divisional of U.S. patent application Ser. No.13/864,466, filed Apr. 17, 2013, now U.S. Pat. No. 10,026,511, whichclaims the benefit of U.S. Provisional Application No. 61/625,226 filedApr. 17, 2012, the entire disclosures of which are incorporated byreference herein.

BACKGROUND

The following relates to the nuclear power reactor arts, nuclearreaction coolant system arts, nuclear power safety arts, and relatedarts.

Light water nuclear reactors are known for maritime and land based powergeneration applications and for other applications. In such reactors, anuclear reactor core comprising a fissile material (for example, ²³⁵U)is disposed in a pressure vessel and immersed in primary coolant water.The reactor core heats the primary coolant in the pressure vessel, andthe pressure vessel includes suitable devices, such as heaters andspargers, for maintaining the primary coolant at a designed pressure andtemperature, e.g. in a subcooled state in typical pressurized waterreactor (PWR) designs, or in a pressurized boiling water state inboiling water reactor (BWR) designs. Various vessel penetrations takeprimary coolant into and out of the pressure vessel. For example, insome PWR designs primary coolant is passed through large-diameterpenetrations to and from an external steam generator to generate steamfor driving a turbine to generate electrical power. Alternatively, anintegral steam generator is located inside the reactor pressure vessel,which has advantages such as compactness, reduced likelihood of a severeloss of coolant accident (LOCA) event due to the reduced number and/orsize of pressure vessel penetrations, retention of the radioactiveprimary coolant entirely within the reactor pressure vessel, and soforth. Additional smaller diameter vessel penetrations are provided toadd primary coolant (i.e., a makeup line) or remove primary coolant(i.e., a letdown line). These lines are typically connected with anexternal reactor coolant system inventory purification device (RCI) thatmaintains a reservoir of purified primary coolant. Further vesselpenetrations may be provided to connect with an emergency condenser, orfor other purposes.

Light water reactors must be evaluated to determine their response inthe event that a pipe outside of the reactor vessel breaks and a loss ofcoolant accident (LOCA) occurs. The compact integral reactor design wasdeveloped, in part, to minimize the consequence of an external pipebreak. However, the integral reactor designs still utilize small boreconnecting piping that transports reactor coolant to and from thereactor vessel. Breaks in these pipes can cause a LOCA, and must beevaluated as design basis accidents.

BRIEF SUMMARY

In accordance with one aspect, a nuclear reactor comprises: a nuclearreactor core comprising a fissile material; a pressure vessel containingthe nuclear reactor core immersed in primary coolant disposed in thepressure vessel; and an isolation valve including a mounting flangesecured to a wall of the pressure vessel and a valve body disposed inthe wall or in a flange assembly including the mounting flange, theisolation valve closing responsive to a pressure difference across thevalve exceeding a threshold pressure difference.

In accordance with another aspect, a system comprises: at least onecoolant pump configured to pump coolant water into or out of anassociated nuclear reactor vessel; at least one external coolant conduitconnecting said at least one coolant pump with the associated nuclearreactor vessel; and a vessel isolation valve having a mounting flangeconfigured to connect with a mating flange of a vessel penetrationthrough an outer wall of the associated nuclear reactor vessel. Thevessel isolation valve fluidly connects with the at least one externalcoolant conduit. The vessel isolation valve is configured to blockoutward flow from the pressure vessel when a pressure differentialacross the valve exceeds prescribed criteria. The vessel isolation valvefurther includes: a valve seat defined in the mounting flange; amoveable valve member movable between an open position permitting flowthrough the vessel isolation valve and a closed position seating againstthe valve seat to block flow through the vessel isolation valve; and abiasing member that biases the valve member towards the open position.

In accordance with another aspect, an isolation valve comprises: amounting flange configured to connect with a mating flange of a pressurevessel; a valve seat formed into the mounting flange; and a valve membermovable between an open position permitting flow through the isolationvalve and a closed position in which the valve member seals against thevalve seat to block flow through the isolation valve. The valve memberis disposed inside the mounting flange or is arranged respective to themounting flange so as to be disposed inside the mating flange of thepressure vessel when the mounting flange is connected with a matingflange of a pressure vessel. The isolation valve optionally furthercomprises a biasing member operatively connected to the valve member tobias the valve member towards the open position. If included, thebiasing member is suitably configured to provide bias effective to keepthe valve member in the open position except when a differential fluidpressure across the isolation valve and directed outward from thepressure vessel exceeds a threshold pressure.

In accordance with another aspect, an apparatus comprises a pressurevessel including a mating flange and an isolation valve as set forth inthe immediately preceding paragraph whose mounting flange is connectedwith the mating flange of the pressure vessel. In accordance withanother aspect, an apparatus comprises a nuclear reactor comprising (i)a pressure vessel including a mating flange and (ii) a nuclear reactorcore comprising fissile material disposed in the pressure vessel, andfurther includes an isolation valve as set forth in the immediatelypreceding paragraph whose mounting flange is connected with the matingflange of the pressure vessel of the nuclear reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for purposes of illustratingpreferred embodiments and are not to be construed as limiting theinvention.

FIG. 1 is a schematic diagram of an exemplary reactor coolant system.

FIG. 2 is a perspective view of an exemplary vessel isolation valve.

FIG. 3 is a perspective cutaway view of the isolation valve of FIG. 2.

FIG. 4 is a cross-sectional view of another exemplary isolation valve.

FIG. 5 is a perspective cutaway view of another exemplary isolationvalve.

FIG. 6 is a perspective view of the valve of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of a nuclear reactor including apressure vessel 10. The pressure vessel 10 contains a nuclear reactorcore 11 (shown in phantom) disposed at or near the bottom of thepressure vessel 10 and immersed in primary coolant water also disposedin the pressure vessel 10. The pressure vessel 10 further containsnumerous internal components that are not visible in FIG. 1 but whichare known in the art, such as structures defining a primary coolant flowcircuit, e.g. a hollow cylindrical central riser defining a hot leginside the riser and a cold leg in a downcomer annulus (e.g., flowregion) defined between the central riser and the pressure vessel 10,and neutron-absorbing control rods and associated drive mechanisms forcontrolling reactivity of the nuclear reactor core. Some embodiments,e.g. integral pressurized water reactor (PWR) designs, also include oneor more steam generators disposed inside the pressure vessel, typicallyin the downcomer annulus.

A reactor coolant system inventory purification device (RCI) 12 isprovided to maintain the quantity and purity of primary coolant insidethe pressure vessel. A makeup line 14 delivers primary coolant from theRCI 12 to the pressure vessel 10, and a letdown line 16 removes primarycoolant from the pressure vessel 10 into the RCI 12. The RCI 12 includesa pump 17 and other water processing components (not shown) forpurifying and storing reserve primary coolant, injecting optionaladditives such as a soluble boron compound (a type of neutron poisonoptionally used to trim the reactivity), or so forth. Integral isolationvalves 20 are provided at respective vessel penetration locations wherethe makeup line 14 and letdown line 16 pass through an outer wall 18 ofthe pressure vessel 10. The integral isolation valves 20 are configuredto control flow into and/or out of the pressure vessel 10 through themakeup line 14 and letdown line 16 such that, during a LOCA, flow ofcoolant out of the pressure vessel 10 is automatically blocked.

Turning to FIGS. 2 and 3, an exemplary integral vessel isolation valve20, hereinafter referred to as simply an isolation valve 20, generallyincludes a biased open axial flow stop valve that is bolted or otherwisesecured to a vessel penetration flange 22 providing a fluid penetrationthrough the outer wall 18 of the reactor pressure vessel 10. As seen inFIG. 2, the exemplary isolation valve 20 generally includes a valve body24 having a mounting flange 28 for securing the valve body 24 to themating flange 22 of the reactor pressure vessel 10. The mounting flange28 includes a plurality of bolt holes 32 for receiving bolts 36 or otherfasteners for securing the valve body 24 to the flange 22 of the reactorpressure vessel 10. The flange 22 of the pressure vessel 10 may be insetinto or flush with the wall 18 of the pressure vessel 10, or may extendoutward as shown in FIGS. 2 and 3, e.g. a forging or casting integrallyformed with the wall 18 or welded to the wall 18. The flange 22 hascorresponding holes for receiving fasteners (e.g., bolts 36). Suitablesealing elements, such as gaskets or o-rings, can be provided forsealing the connection of the flanges 22, 24.

As best seen in FIG. 3, which is a cross-sectional view of the isolationvalve 20 taken through a central longitudinal axis thereof, theexemplary isolation valve 20 includes a valve member in the form of apiston 38 that is supported within the valve body 24 for reciprocatingaxial movement. The piston 38 is configured to seal against a valve seat42 that is generally formed by a radially-inwardly extending shoulderwithin a central bore 44 of the valve body 24. When mounted to thepressure vessel 10, the piston 38 and valve seat 42 are located insidethe wall 18 of the reactor pressure vessel 10, or at least inside theflange 22 extending away from the wall 18 (in some embodiments in whichthe flange protrudes away from the wall 10). The function of theisolation valve 20 is to seal the penetration 22 in the reactor pressurevessel 10 in the event of a pipe break or other sudden loss of pressurein the pipes external to the reactor pressure vessel 10, such as makeupline 14 and/or letdown line 16. Such sealing thus occurs at or within anouter boundary of the pressure vessel 10.

As noted, the valve body 24 is generally hollow and has internal bore 44extending from a first axial end to a second axial end thereof. Thefirst axial end of the valve body 24 is configured to be received withinthe penetration 22 and serves as a flow inlet or outlet 54 (depending onthe direction of flow through the isolation valve 20). The end 54 may bean open end in fluid communication with the downcomer annulus or otherplenum defined inside the pressure vessel 10, or may connect to internalpiping or other flow passages (not shown) within the pressure vessel 10.The second axial end of the valve body 24 is enclosed by a spring cover56 which, as will be described in more detail below, houses a spring 58for biasing the piston valve member 38 to an open position.

An opening 60 in the valve body 24 communicates with the central bore 44such that fluid can flow between the central bore 44 and the opening 60.The opening 60 functions as a flow inlet or outlet (again, depending onthe direction of flow through the isolation valve 20). The opening 60can be connected to suitable piping, such as makeup line 14 or letdownline 16, depending on its specific application.

As noted above, a movable valve member in the form of piston 38 issupported within the central bore 44 of the valve body 24 forreciprocating axial movement between an open position (as shown in FIG.3) permitting flow through the bore 44, and a closed position whereatthe piston 38 seals against the valve seat 42 within the central bore44, thus blocking flow through the central bore 44. In the illustratedembodiment, the valve seat 42 is generally coaxially aligned with themounting flange 28 such that when the valve body 24 is secured to apressure vessel, the valve seat 42 is disposed within the interior ofthe pressure vessel 10, or within the wall 18, or within the vesselpenetration flange assembly 22, 28 (that is, an assembly of securedflanges 22, 28 that does not include any intervening piping).

A rod 64 is connected with the piston 38 and extends axially through thecentral bore 44 and protrudes from the second axial end of the valvebody 24. The protruding end of the rod 64 is operatively connected to abiasing member in the form of the spring 58 that is contained withinspring cover 56. Spring 58 is in compression and acts between the springcover 56 and the rod 64 to bias the piston 38 towards the open positionas shown in FIG. 3. Absent any flow through the central bore 44, thespring 58 generally maintains the piston 38 in its open position.

During normal operation of the isolation valve 20, the spring 58 isconfigured to maintain the piston 38 in its open position to permit flowbetween the axial inlet/outlet 54 in the valve body 24 and theinlet/outlet 60 of the valve body 44. That is, the spring 58 isconfigured to apply enough force to the piston 38 to permit a desiredamount of flow through the valve 20 in either direction during normaloperation.

However, if the pressure on the inside of the pressure vessel 10 exceedsa prescribed pressure threshold, such pressure acting on piston 38 willovercome the preload spring bias thereby shifting the piston 38 to theclosed position sealing the piston 38 against valve seat 42 andpreventing flow from the interior of the pressure vessel through thepenetration 22.

It should be appreciated that, when installed on a pressure vessel 10,the valve 20 operates to seal the penetration 22 automatically when thepressure differential across the valve exceeds a threshold value. Thethreshold value can be set at least in part by the amount of biasapplied to the piston 38 by spring 58. For example, during normaloperation the pressure differential across the valve 20 may be minimal,and the spring 58 therefore would act to keep the piston 38 in the openposition. In some instances, the pressure differential across the valvemay increase. For example, a break in an external pipe connected to thevalve 20 would result in a decrease in the pressure at inlet/outlet 60.If this decrease in pressure is large enough, it may have the effect ofincreasing the pressure differential between inlet/outlet 54 andinlet/outlet 60 such that the bias applied to the piston 38 by spring 58would be overcome and thereby shift the piston to the closed position.The piston 38 would then remain in the closed position as long as thepressure differential that shifted the valve to the closed statecontinues to exist.

It will be appreciated that spring 58 can be configured to providedifferent levels of biasing force to the piston 38. For example, aplurality of springs having various spring constants can be provided,and a given spring chosen and installed in to the valve 20 depending ona particular application. Alternatively, or in addition, a springpreload mechanism can be used to apply a preliminary preload to spring58 to vary the force applied to the piston 38. Such a spring pretensionmechanism can include a spring cover 56 having an axial length that isless than the axial length of the spring 58 such that, when installed,the spring cover 56 compresses the spring 58. By providing spring coversof different axial length, more or less preload can be applied to agiven spring.

Since the spring and spring cover of the valve shown in FIGS. 2 and 3are located external to the pressure vessel 10 when the valve 20 isinstalled thereon, conventional materials can generally be used for thespring element and/or spring cover. The spring cover 56 and spring 58are not safety-critical, because failure or removal of the spring cover56 and/or spring 58 would simply remove the bias force keeping theisolation valve open and cause the valve to close to prevent coolantfrom escaping from the pressure vessel. (Alternatively, if there ispositive flow from the opening 60 to the inlet/outlet 54 the valve maystay open in spite of the failure of the biasing mechanism 56, 58; but,in that case again no fluid would escape from the pressure vessel 10because the flow would be into the vessel 10). It should be appreciated,however, that the spring and/or portions thereof could extend into thepressure vessel 10 or into the wall 18 or flange 22, or be locatedentirely within the pressure vessel.

Turning to FIG. 4, another exemplary isolation valve 80 is illustratedhaving a spring element that is configured to be supported within apressure vessel wall 82 (or within a welded flange extending therefrom)when installed. In this embodiment, the valve 80 is supported within agenerally cylindrical valve body 84 that includes a thermal sleeve 86.The valve body 84 is supported within a bore 88 extending through thevessel wall 82. An annular space is formed between the thermal sleeve 86and the bore 88 in which primary coolant can circulate for reducingthermal shock to fluid flowing into the reactor pressure vessel 82.

In the illustrated embodiment, the valve body 84 is welded or otherwisesecured to a mounting flange 90 that is configured to be bolted to amating flange 92 that is flush with, inset into, or protrudes from thepressure vessel wall 82. A central passageway 98 in the mounting flange90 communicates with a central passage 102 of the valve body 84.Supported within the central passage 102 of the valve body is a moveablevalve member 108 that is biased to an open position by compressed spring112. In this regard, spring 112 is interposed between radially inwardlyextending ribs or vanes 113 of the valve body 84 and a spring retainingribs or vanes 114 of the valve member 108 such that the spring biasesthe valve member 108 to the left in FIG. 4 and maintains it in an openposition during normal operation. The moveable valve member 108 includesa sealing head 110 that is adapted to seal against a generally conicalvalve seat 118 formed in the flange 90, as will be described.

Under normal operating conditions, the valve is configured to permitflow into the pressure vessel through the valve 80 from external piping,such as makeup line 14 shown in FIG. 1, with the spring 112 and pressureof the fluid flowing into the pressure vessel 82 acting to maintain thevalve member 108 in the open position. In this open position, the ribsor vanes 113, 114 are spaced apart or have fluid passageways so thatfluid can flow into the pressure vessel via the central passageway 98and the thermal sleeve 86. Should the pressure differential (in theoutward direction) across the valve 80 exceed the spring bias applied tothe valve member 108, such as when a break in the external piping isexperienced (or when pressure inside the vessel 82 increases), thepressure in the pressure vessel 82 will act to compress the spring 112and the valve member 108 will shift to the right from its position inFIG. 4 and seal against valve seat 118. This effectively seals thepenetration through pressure vessel wall 82 in the event of a LOCA, andthe valve will generally remain in the closed position until thepressure differential that resulted in the closing of the valve ismitigated.

It will further be appreciated that the illustrated valve can alsoaccommodate fluid flowing out of the pressure vessel 82 through thevalve 80, such as letdown flow flowing into letdown line 16. In such aconfiguration, the spring bias maintains the valve member in the openposition against the pressure of fluid flowing out of the pressurevessel 82. A sudden increase in pressure within the pressure vessel 82,or a drop in pressure of fluid in the letdown line (due to a break, forexample) such that the pressure differential in the outward directionexceeds a closure threshold will result in the valve member 108 shiftingto the closed position. Various springs can be used to provide a valvewith desired operating characteristics. For example, in someapplications a relative high bias open force may be desired, and a verystiff spring can be utilized to achieve the desired bias.

The thermal sleeve 86 provides protection against thermal shock, and isadvantageous for the isolation valve protecting the make-up line 14. Onthe other hand, thermal shock to the nuclear reactor is less likely tooccur in the case of the let-down line 16. Accordingly, in someembodiments the thermal sleeve is omitted. In such cases, the inner wallof the bore 88 extending through the vessel wall 82 suitably serves asthe anchor for the isolation valve. Alternatively, a sleeve structurallysimilar to the thermal sleeve 86 may be provided, but made of thermallyconductive material and/or without any gap between the sleeve and thebore 88. It should also be noted that the valve seat 118 is a surface ofthe flange 90, and accordingly proper sealing of the isolation valve isnot dependent upon the structural strength or integrity of the sleeve86.

In some embodiments, it may be desired to provide additional closureforce to assist and/or ensure closure of the valve. For example, withreference back to FIG. 3, an electrical, pneumatic or hydraulic actuatorpiston P (shown in phantom) can be connected with the end of the rod 64.When working fluid is injected into the actuator piston P via actuationline A, the actuator piston acts via the rod 64 to actively pull thepiston 38 into closure against the valve seat 42. A solenoid withelectrical actuation could be substituted for the actuator piston P andfluid actuation line A.

As another option, a latch disposed on the rod 64 can engage a lockingmechanism (features not shown) when piston 38 moves to the closedposition so as to lock the piston in the closed position. The lockingmechanism can include a manual or electrically actuated release so thatthe isolation valve only re-opens when the operator activates therelease. This arrangement ensures that the isolation valve cannotre-open prematurely (that is, before the operator intends for it toreopen).

The isolation valves of FIGS. 2-4 provide bidirectional operation, andonly close upon an outward pressure differential greater than athreshold value. Such isolation valves can be used for both makeup lines14 and letdown lines 16. However, if the vessel penetration is intendedto support only unidirectional flow into the pressure vessel, then thevalve can modified accordingly.

With reference to FIGS. 5-6, another isolation valve embodiment includestwo valves 80, 80 a aligned in series, wherein the inner valve 80 islocated inside wall of the pressure vessel and the outer valve 80 a islocated inside a wall forging 72 that is external to the pressure vessel10 such that it may be bolted to the pressure vessel 10.

The disclosed isolation valves passively isolate vessel penetrationsthat are in fluid communication with the reactor coolant in the pressurevessel in the event of a pressure rise inside the pressure vessel. Asshown in FIG. 1, these isolation valves are suitably used on makeup andletdown lines. Additionally, they may find application in protectingother vessel penetrations, such as the return line of an emergencycondenser. Because the disclosed isolation valves are located inside thepressure vessel wall or inside a welded vessel penetration flange or ina flange assembly, they are failsafe against pipe breakages. (Aspreviously noted, in the embodiment of FIGS. 2 and 3 although thebiasing elements 56, 58 are outside the pressure vessel and flangeassembly the valve components, e.g. piston 38 and valve seat 42, wouldcontinue to operate to provide failsafe operation even if the biasingelements 56, 58 are broken off). Although not illustrated, in theembodiment of FIG. 4 or FIG. 5 the flange 90 may optionally be a spoolflange enabling a flanged connection of an external component to theflange 90. In this case, there is no external piping between theexternal component and the vessel penetration, and the vesselpenetration is protected by the isolation valve.

The disclosed vessel isolation valves are well-suited for use inconjunction with protecting a vessel penetration in a nuclear reactorpressure vessel. However, the disclosed vessel isolation valves are moregenerally applicable to protecting a vessel penetration in a pressurevessel generally, i.e. in a pressure vessel for an application otherthan housing a nuclear reactor.

The preferred embodiments have been illustrated and described.Obviously, modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

We claim:
 1. A system comprising: at least one coolant pump configuredto pump coolant water into or out of an associated nuclear reactorvessel; at least one external coolant conduit connecting said at leastone coolant pump with the associated nuclear reactor vessel; and avessel isolation valve having a mounting flange configured to connectwith a mating flange of a vessel penetration through an outer wall ofthe associated nuclear reactor vessel, the vessel isolation valvefluidly connecting with the at least one external coolant conduit, thevessel isolation valve configured to block outward flow from thepressure vessel when a pressure differential across the valve exceedsprescribed criteria; wherein the vessel isolation valve furtherincludes: a valve seat defined in the mounting flange, a moveable valvemember movable between an open position permitting flow through thevessel isolation valve and a closed position seating against the valveseat to block flow through the vessel isolation valve, and a biasingmember that biases the valve member towards the open position.
 2. Thesystem of claim 1, wherein the valve member of the vessel isolationvalve includes a piston adapted to seal against the valve seat.
 3. Thesystem of claim 2, wherein the vessel isolation valve further comprises:a rod connected to the piston and protruding axially away from themounting flange, and wherein the biasing member comprises a springoperatively engaged with the rod.
 4. The system of claim 3, wherein thespring is at least partially surrounded by a spring cover removablysecured to the vessel isolation valve.
 5. The system of claim 1, whereinthe movable valve member is entirely contained in one or more of (i) theassociated nuclear reactor vessel, (ii) the outer wall of the associatednuclear reactor vessel, and (iii) a flange assembly including a flangewelded to the outer wall and the mounting flange of the vessel isolationvalve.
 6. The system of claim 1, wherein the vessel isolation valvefurther includes at least one of a latching device and an actuatoroperatively coupled to the movable valve member for maintaining thevalve member in the closed position.